Add supports for pulp notification callbacks

* adds /api/repositories/events POST action
* adds PulpEventHandler class to parse and act on the events.
* adds an option to auto register events on pulp, this however is dead code
at the moment since i would assume the workflow will change once we have
multi-pulp support

to register notifications (in the meanwhile) manually on your pulp
server execute:
pulp-admin event listener http create